This invention relates to a vacuum circuit interrupter of the type comprising a metallic housing defining an arc extinguishing chamber, a pair of contacts, one stationary and one movable, disposed in the housing, and at least on electrical insulating chamber secured to at least one end of the arc extinguishing chamber.
As all vacuum circuit interrupters are operated in a vacuum they can be expected to have the advantages including the following:
1. Their overall dimension is decreased because a very small distance of separation between the contacts permits the interrupter to withstand high voltages.
2. This decrease in dimension leads to both the simplification of the associated operating mechanism and an increase in the response speed thereof.
3. Because of the high speed at which ions produced during switching are diffused and extinguished immediately after an interruption of current flow, the interrupter is rapidly restored to its insulating state to withstand any restriking voltage, thus ensuring that it can rapidly and effectively interrupt a high current flowing therethrough while improving the reliability of the device.
4. Both of the contacts or electrodes are maintained in a clean state at all times and are capable of always engaging each other without oxidizing, thereby permitting a high current to readily flow therethrough. In addition, there is no fear that the interior of the exhausted vacuum envelope may be contaminated to deteriorate the capability of the interrupter. Thus a long useful life is ensured without any maintenance.
Vacuum circuit interrupters having the above cited advantages are described and claimed, for example, in U.S. Pat. Nos. 3,280,286, 3,231,705 and 3,082,307. The conventional vacuum circuit interrupters include an arc extinguishing chamber defined by a cylindrical metallic housing having a pair of contact members disposed therein, and an electrical insulator in the form of a hollow cylinder disposed at one or both ends of the housing. It has been a common practice in the production of such devices to heat both the arc extinguishing chamber and the insulator or insulators to the same temperature by a common heating device during the exhausting operation. On the other hand, during the operation of the vacuum circuit interrupters the components within the arc extinguishing chamber including the pair of contact members, that is to say, those portions contacting the so-called interrupting arc, are raised to the highest temperature as compared to the remaining portions of the device. Thus, it is desirable to heat those portions to a temperature substantially higher than the temperatures of the remaining portions of an interrupter or to a temperature equal to at least the said highest temperature during the exhausting operation while the latter operation is performed for a long interval of time until the interior of the interrupter reaches the desired high vacuum. Under these circumstances the insulator providing an electrical insulating chamber should be maintained at a temperature fairly lower than the temperature at which the above-mentioned portions are heated because of its low thermal resistance.
Furthermore, during the combined heating and exhausting operation and in the operation of vacuum circuit interrupters, a metallic vapor or vapors and scattered particles that may be formed within the arc extinguishing chamber can freely enter the associated insulating chamber to contaminate the latter. Also with the metallic housing heated to an elevated temperature, the insulating chamber can be also heated to an elevated temperature principally due to the thermal radiation from the housing. Practically, the metallic housing has been precluded from being heated to an elevated temperature above a temperature up to which the insulating chamber is allowed to be heated.
Additionally, in order to prevent an extraordinary voltage from occurring across the pair of contacts due to the chopping of a current upon interruption as well as to prevent the contacts from fusing to each other, it has been proposed to include a low melting-point metal such as bismuth, tellurium, antimony, silver or the like in the material for the contacts as disclosed in U.S. Pat. Nos. 2,975,255 and 3,246,979. Thus, if either or both of the contacts including a low melting-point metal is or are used in vacuum circuit interrupters, the metal can be readily evaporated when the associated arc extinguishing chamber has been heated to an elevated temperature and when an electric arc has been established across the contact member. When the housing and shields are formed of iron or nickel as in the prior art devices, the evaporated material adheres thereto and as a result the low melting-point metals are generally characterized as having a decreased ability to withstand voltages. Therefore in order to improve the characteristics of the prior art interrupters, it is highly desirable to select the material and construction of a housing providing an arc extinguishing chamber and shields so that the material is hardly affected with a vapor of such a low melting-point metal. This material should also have the desirable quality that it resists the deformation of the housing when it is subjected to a high vacuum at an elevated temperature. As a result the useful life of the interrupter will increase and the voltage characteristics will improve.
Interrupters having these improved qualities have not been previously put to practical use due to the following problems:
1. It has been found heretofore that stainless steel is unsuitable for effectively accomplishing the cooling of electric arcs, the condensation of metallic vapors and the sticking of particles spattered from the metallic material of the electric contacts. In order to enable the housing and/or arc shield to effectively accomplish the condensation and sticking respectively of the metallic vapor and spattered particles originating from the contact metal, the material of the housing has been considered preferably to have the following properties:
(a) it should be high in thermal conductivity; and
(b) it should be the same as the material of the contacts.
Regarding the property (a), it is well known that for current interrupters high in interrupting current capacity, metals high in thermal conductivity are more suitable than electrically insulating materials such as glass and ceramics etc. This can readily be understood from the fact that the material of the housing and/or arc shield which is higher in thermal conductivity has a larger coefficient of condensation of metallic vapor resulting in the electric arc being more effectively cooled. Regarding the property (b), engineering principles with respect to evaporation teach that if the housing and/or arc shield is formed of a metal identical to that of the vapor thrown thereon from the associated contacts, its coefficient of condensation has a maximum value. The foregoing is applicable to the sticking of the spattered particles from the contact metal.
It is absolutely essential for vacuum circuit interrupters that the spattered metallic particles from the contact metal must rigidly adhere to the housing and/or arc shield during service. More specifically, vacuum circuit interrupters perform the switching operation through the impulsive engagement and disengagement of the movable contact with and from the stationary contact by the operation of the bellows involved. If the spattered metallic particles originating from the contacts do not rigidly adhere to the housing and/or arc shield, then the metallic particles thereon can peel off from the housing and/or arc shield due to the impulsion developed in the switching operation. This may lead to malfunctions wherein the contacts in their open position will be short-circuited. Furthermore, if the metallic particles do not adhere rigidly on the housing and/or arc shield, a disadvantage results in that the vacuum circuit interrupter deteriorates in its voltage withstanding property.
On the other hand, it has been generally practiced to use copper, as the base metal for contact materials, together with a low melting point metal(s) for the purpose of increasing the interrupting current capacity. Examples of the low melting point metal include bismuth, and tellurium, etc., and such metals serve to improve both the anti-fusion and the chopping current characteristic.
For the above reasons, the concept has prevailed that stainless steel is unsuitable for foming the housing and/or shield for vacuum circuit interrupters required to meet the conditions that (a) the material of the housing and/or arc shield be high in thermal conductivity; (b) the housing and/or arc shield should provide both a high coefficient of condensation of metallic vapor caused from the contact material and a surface upon which the spattered metallic particles can adhere rigidly; and (c) the contacts formed of copper, as the base metal, together with a low melting point metal(s) for example, bismuth or tellurium or the like. Therefore, vacuum circuit interrupters of the prior art type have comprised a housing and/or arc shield formed of copper, nickel, iron or the like. It is apparent that such metals are high in thermal conductivity, coefficient of condensation of metallic vapor and sticking force of spattered metallic particles, as compared with stainless steel.
(2) It has been believed that stainless steel is unsuitable for forming evacuated housings also serving as the arc-extinguishing compartments because such stainless steel is corroded when contacted by molten bismuth. This fact is the subject of the publication of the Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Vol. 9, entitled "Reactor Technology and Chemical Processing". Therefore stainless steel has been previously considered as impractical for use with the evacuated housing for vacuum circuit interrupters required to have a long useful life.
Thus, while vacuum circuit interrupters have been developed whose housings are made of stainless steel, which housings are evacuated to serve as arc-extinguishing compartments, those circuit interrupters have included special means for minimizing a thermal input to the housing and/or arc shield. For example, external coil or permanent magnet means have been necessarily used to establish a magnetic field coaxial with a flow of arcing current thereby to magnetically drive the electric arc so as to confine it as much as possible within a space formed between the opposed contacts, while at the same time metallic particles spattered from the contacts are prevented from reaching the outer peripheral portion around the contacts, that is, the housing and/or arc shield.
In other words, in the prior art, the formation of the housing and/or arc shield of stainless steel has been accompanied by the provision of means for establishing a magnetic field operative to prevent the particular metallic vapor from being scattered toward the housing and/or arc shield for the purpose of limiting the thermal input to the housing and/or arc shield to a certain magnitude. The provision of such additional means renders the resulting circuit interrupter excessively expensive, so that there has been a tendency to avoid the formation of the housing and/or arc shield of stainless steel. To this end, the arc-extinguishing compartment has been heretofore formed of any suitable electrical insulating material. Alternatively, if it is required to be formed of a metallic material, iron or copper has been mainly used. In addition to the tendency to avoid the formation of the housing and/or arc shield of stainless steel as above described, the idea has never been expressed that the electric contacts should include a low melting point metal used in conjunction with the arc-extinguishing compartment formed of stainless steel.
However, applicants have found that low melting point metals such as bismuth, at temperatures above approximately 400.degree. C., do not adhere rigidly to stainless steel. Also, applicants have found that even at relatively high temperatures, stainless steel is higher in mechanical strength than iron and copper thus permitting the housing to be actually heated at a temperature of from 450.degree. to 900.degree. C. and evacuated and that it is advantageous in that its weldability is good, while providing good anti-fusion and chopping current characteristics.
As a result of experiments it has been found that in vacuum circuit interrupters comprising the combination of a housing and/or arc shield formed of stainless steel, and electric contacts including a low melting point metal, the above-mentioned disadvantages have been negligible for all practical purposes and therefore the interrupters can be satisfactorily put to practical use. More specifically, with the housing or arc shield formed of stainless steel, heavy currents whose magnitude exceeded 40 KA were successfully interrupted by properly selecting the thickness of the stainless steel its diameter relative to that of the contact, its surface roughness and means for cleaning its surface, etc. Furthermore, it has been found that when the housing and/or arc shield are roughly polished, the adherence of the spattered metal on the housing and/or arc shield has been improved to such an extent that the sticking force has not caused any practical problems.
With the contacts including copper as the base metal, practical experiments have been conducted in terms of the corrosion of stainless steel effected with a small amount of melted bismuth present in the copper, due to the melted metallic particles spattered from the contacts and stuck to the housing and/or arc shield. The results of the experiments indicate that the portion of bismuth dissolved or penetrated into the stainless steel is substantially negligible.
During the manufacture of vacuum circuit interrupters the housing and/or arc shield of stainless steel is subjected to heating, evacuation, and outgassing processes, by which a low melting point metal such as bismuth, tellurium or the like contained in the contacts may be heated and evaporated. It has been found that the evaporated portion of bismuth is not readily deposited on a housing and/or arc shield made of stainless steel. Therefore, it is very difficult for the stainless steel to become corroded with bismuth. Furthermore, the housing and/or arc shield of stainless steel are excellent in voltage withstanding properties.